[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[wim glenn]

Change by wim glenn :


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[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[anthony shaw]

anthony shaw  added the comment:

Hi Peter, this proposal would be easier to review as a GitHub Pull Request.

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[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[Peter Eckersley]

Peter Eckersley added the comment:

OK, here's another solution following paul.j3's suggestion. I think this is 
much better:

https://gist.github.com/pde/817a00378d3f6ed73747dfffce323ae5

Tests & documentation included.

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[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[Peter Eckersley]

Peter Eckersley added the comment:

Thanks for your feedback Paul! I agree your proposed implementation strategy 
would probably be saner; I'll revise the patch to use that approach or 
something like it.

As for the question of necessity, there are definitely more cases than just the 
store_false ones -- I documented these in the linked Certbot bug but very 
briefly they include:

* Programs where the default value of a variable is "Ask interactively" if no 
flag is provided
* Cases where the default value is the result of some complicated computation 
(for instance, reading it from a defaults file)

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[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[paul j3]

paul j3 added the comment:

I might accept a patch that tweaks the ArgumentDefaultsHelpFormatter class, but 
adding a parameter that must propagate through all the Action classes in just 
plain wrong.  

Users are confused by the many Action parameters as it is.  And based on 
StackOverFlow questions, I'd say there are lots of custom Action classes.  We 
cannot make a change that might break those!

One possibility is to tweak the `%(default)s` test to something like:

 if '(default` not in action.help:

Then the user who is not happy with the '%(default)s` display could just write 
a help like:

 help = 'my help text (default: mycustomvalue)'

In other words, embed the bypass mechanism entirely in the 'help' string and 
the Formatter class.  The mechanism would have to be something that is simple 
to explain, and be unlikely to interfere with existing uses of this Formatter.

And to play it safe, I'd suggest field testing a SmarterDefaultsHelpFormatter 
class first - one that implements a change like this, without touching the 
existing class.

Keep in mind that ArgumentDefaultsHelpFormatter is never essential.  The 
argparse developer can always add the '%(default)s` strings to selected 'help' 
lines.  He can even do this in utility functions that know more about the 
underlying program semantics.  I don't think this proposed enhancement gives 
the end user any added control.  

If you want to make a stronger case for this enhancement, find other cases 
where it would be useful.  So far you are just arguing that '(default= True)' 
for a 'store_false' Action may convey the wrong sense to users.  As 
r.david.murray argued, this problem can be minimized with a proper phrasing of 
the help string.

Another thought - 'store_false' is just a subclass of 'store_const'.  You could 
use that class instead with a different set of 'const' and 'default' values. 
e.g. ('Yes','No', or 'not-False', 'not-True').

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[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[Peter Eckersley]

Peter Eckersley added the comment:

Patch is now against the latest Python development tree, and includes test 
cases:

https://gist.github.com/pde/daec08cadc21bca0d62852020887ee13

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[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[Peter Eckersley]

Peter Eckersley added the comment:

One thing I noticed when testing my patch by vendorizing it into the Certbot 
tree was that if a developer had somehow inherited from a version of 
argparse.Action that *doesn't* have this patch applied to it, and then passes 
in instances of those inheriting classes to the new patched version, things 
will break (because the argparse engine now assumes every action has a 
help_default attribute, rather than checking for it).

That's an unlikely situation but might arise if a developer had managed to use 
the standard library version of argparse in some places, and the pypi packaged 
version in other places.

It might be possible, but uglier, to write this patch more defensively so that 
that wouldn't happen; I would appreciate some feedback on whether people think 
that's necessary or a good idea.

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type:  -> enhancement

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[issue28742] argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate documentation of defaults, so they should be overrideable

[Peter Eckersley]

New submission from Peter Eckersley:

When argparse lists the default values for cli flags and arguments, it shows 
argparse's view of the internal Pythonic default, not the default behaviour of 
the program as a whole. This can be wrong in many cases, as documented at 
https://bugs.python.org/issue27153 and 
https://github.com/certbot/certbot/issues/3734.

To mitigate this, we should allow the caller to set arbitrary strings that 
argparse will display to the user as the "default" in the CLI help.

I wrote a first patch to do this via a new "help_default" argument that can be 
added to argparse actions:

https://gist.github.com/pde/daec08cadc21bca0d62852020887ee13

The patch was written against argparse.py from Python2.7, but seems to apply 
and run okay against the Python 3.x version.

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messages: 281183
nosy: pde
priority: normal
severity: normal
status: open
title: argparse.ArgumentDefaultsHelpFormatter sometimes provides inaccurate 
documentation of defaults, so they should be overrideable
versions: Python 3.7

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